Many batteries generate combustible gases during operation. These gases are either vented from the battery container into the atmosphere or recombined within the battery in secondary reactions with the active materials. However, even in batteries which provide for internal recombination of combustible gases, there are certain circumstances, such as inadvertent or abusive overcharge, in which the recombination mechanism is ineffective and significant volumes of combustible gases are generated.
Combustible gases within the head space of a battery may be accidentally ignited and result in an explosion. The damage and injury resulting from such explosions are well documented. Thus, for many years, effective and reliable means have been sought for preventing or minimizing explosions in batteries and the hazardous effects thereof.
Combustible gases which are generated within a battery, if not effectively recombined, will eventually create a high internal pressure. To alleviate this pressure, these gases must be vented to the atmosphere. Venting is typically accomplished through the use of a simple open vent slot or a one-way relief valve, sometimes referred to as a "burp" valve. During venting of combustible gases, an external source of ignition, such as a flame or spark near the battery vent, can result in an ignition which will propagate back into the battery container and result in an explosion. Improvements in relief valve construction and the development of flame arrestors used in conjunction with vents, have considerably decreased the incidence of battery explosions caused by external ignition sources, provided that such protective devices have not been removed or disabled, or that the integrity of the container or cover has not otherwise been breached.
However, should an external source of ignition breach one of the protective devices, or should an ignition occur within the container, the combustible gases in the head space may explode. The concentration of gases, typically a mixture of hydrogen and oxygen in a typical lead-acid battery, and the relatively large volume of the head space can result in an explosion which will shatter the container, cover or other components. In addition, the explosion will also often carry with it the liquid acid or other hazardous electrolyte from within the container.
Thus, materials and methods for suppressing or minimizing the effects of explosions within batteries have been long sought. Elimination of the open head space, or substantially filling it with a solid material, would virtually eliminate the possibility of an explosion simply because the presence of combustible gases would be eliminated. However, neither alternative is acceptable. An open head space is necessary in virtually all secondary storage batteries. The head space accommodates certain essential battery components, such as plate straps, intercell connectors, or terminals. In addition, in batteries which utilize free liquid electrolyte, sometimes referred to as "flooded" systems, open head space is necessary to accommodate variations in the level of the electrolyte as the battery is cycled, or to provide space for acid movement under extreme conditions of use, such as abusive overcharge. The head space also accommodates movement of the electrolyte level as the battery is tilted in service, such as the ability to operate an automobile on an incline without loss of electrolyte. Thus, due to the need to accommodate certain structural components of the battery and to provide space for electrolyte level fluctuations, the head space in batteries must be maintained.
For many years, it has been known to fill the head space in a battery or cell, either partially or totally, with a porous material to inhibit the explosion of gases within the head space and quench any flame which may be formed, while still allowing the movement of gases and electrolyte through the material. For example, Jensen U.S. Pat. No. 2,341,382 issued February, 1944, discloses partially filling the head space with a loosely packed material, such as crushed stone or glass, diatomaceous earth, or glass wool.
There are a number of factors which are believed to have generally inhibited the practical application of explosion attenuation technology in batteries. These include the creation of other hazards, and detrimental effects on battery performance. As the head space of a battery is filled with a porous material, there will be a decrease in the remaining void volume in the head space inversely proportional to effective void volume of the filler material. In other words, the more solids present in the filler material, the greater will be the reduction in the total head space volume. Particularly in flooded batteries, the loss of actual open head space volume will lessen the space available for electrolyte movement or electrolyte level variations.
It is known that high rate charging or excessive over-charging can result in vigorous gassing in many types of batteries, particularly lead-acid batteries. If the gas bubbles formed in the electrolyte cannot find fairly direct channels to the battery vent openings, electrolyte may be upwardly displaced and overflow through the battery vents. This condition is known as electrolyte pumping or spewing. The damaging and hazardous effects of a corrosive electrolyte flowing out of a battery are obvious.
Electrolyte pumping can also occur even where the head space of the battery is filled with a very highly porous material, i.e., a material having a high void volume. For example, an open cell foam material may have a void volume as high as 97 to 98% and, if placed in the the total volume thereof. Nevertheless, in a flooded battery, such a material may readily retain electrolyte and not allow it to drain back into the battery by gravity. Electrolyte so retained in a porous filler material will be readily pumped from the battery under the conditions of vigorous gassing, described above.
Further, if a relatively large volume of electrolyte is drawn from the cells through wicking by a porous material in the head space, or if the porous material otherwise retains the electrolyte with which it comes into contact, insufficient electrolyte may remain in the cells for proper electrochemical reaction and operation of the battery.
Any material to be used as an attenuation material in batteries must possess certain other physical properties. Such a material must have adequate resilience to retain its shape and to readily fill the sometimes irregular shape of the battery head space. The material must also be thermally and chemically stable in the operating environment within the battery. To provide adequate safety, any attenuation material must be able to survive repeated ignitions without melting or sintering. A material capable of effectively operating only once, which is destroyed in the process would not be satisfactory. Additionally, the material cannot, of course, dissolve in or otherwise react with the liquid electrolyte.
A number of porous plastic materials have been used in fuel tanks or similar containers as a means for reducing explosion hazards. Both fibrous and cellular plastics of various kinds are disclosed in the art. Allen U.S. Pat. No. 3,561,639 issued February, 1971, discloses a single block of open cell polyurethane foam to fill the interior of a fuel tank. The disclosed material has a reticulated (fully open) pore structure, a pore size ranging from 10 to 100 pores per linear inch (ppi), and a void volume of 97%. The fully reticulated structure is described as important to keep flame propagation from reaching the velocity necessary for explosion and to provide a high degree of permeability for the liquid fuel.
Bulked fibrous plastic materials of many types have also been proposed for use as a means of arresting flames and reducing explosion hazards in fuel tanks. The filamentary plastic materials proposed for such use include polyolefins, nylon, dacron, polyesters, acrylics, and polyurethanes, and others. The materials are typically bulked or textured to provide high porosity and void volume by any of many well-known methods such as twisting, looping, crimping, needle punching and so forth. Examples of various types of such materials are described in Stewart U.S. Pat. No. 3,650,431 issued March, 1972, Stanistreet, et al. U.S. Pat. No. 4,141,460 issued February, 1979, and Sheard et al. U.S. Pat. No. 4,154,357 issued May, 1979.
The use of the foregoing porous plastic materials to suppress explosions in fuel tanks, however, does not suggest use of these materials in storage batteries. More particularly, the use of these materials as an explosion attenuation material in the open head space of such batteries is not suggested.
Other patents owned by a common assignee address this problem by providing an explosion attenuating material comprising closely packed pillows made of a foam or a fibrous material such as polypropylene. See Binder et al. U.S. Pat. No. 4,751,154, issued Jun. 14, 1988, and 4,859,546, issued Aug. 22, 1989. The subject matter of these patents is hereby incorporated herein by reference.
In the explosion attenuating materials disclosed in the foregoing patents to Binder et al., certain materials which attenuate explosions and quench the flames resulting from the ignition of combustible gases do not perform well in other aspects of battery operation. The violence of an explosion (in terms of the peak pressure developed within the open head space of a battery) can be reduced by substantially filling the head space with certain types of porous materials. The pressure developed during an explosion is, up to a point, reduced as the pore size of the attenuation material is decreased. Unfortunately, as the pore size of the material decreases, the adverse effects of the material on battery performance increase. The smaller the pore size of the material, the greater the propensity of the material to wick up electrolyte, i.e., to retain within the pores electrolyte with which it is wetted.
Absorbed electrolyte cannot drain back into the cell and can result in two serious problems. First, electrolyte retained in the porous material reduces the level of electrolyte covering the plates, potentially shortening battery life and diminishing electrical performance of the battery. Second, retained electrolyte will inhibit the flow of gases generated within the battery and, in certain circumstances of operation, result in electrolyte being pumped out of the battery through the vent openings. The present invention addresses these problems.